1. Field of the Invention
The present invention relates to a timing synchronization method in a communication system.
2. Description of the Related Art
In general, a timing synchronization method between a transmitter and a receiver is required to receive signals correctly at the receiver in a radio communication system. Especially, in distributed networks like cognitive radio systems, because a receiver does not know when a transmitter will start to send a signal, the receiver needs to acquire a start timing of a transmitted signal for synchronization with the transmitter during receiving the transmitted signal. One of the conventional timing synchronization methods is disclosed by Taira Akinori, Ishizui Fumio and Miyake Makoto, “A Timing Synchronization Scheme for OFDM in Frequency Selective Fading Environment”, The transactions of the Institute of Electronics, Information and Communication Engineers B, Vol. J84-B, No. 7, pp. 1255-1264 (2001). In this reference, a receiver uses a matched filter to acquire a start timing of transmitted signal.
Before starting a communication, a transmitter and a receiver negotiate a signal sequence which has a high auto-correlation with its own signal and low cross-correlations with other signal sequences. Moreover, the transmitter and the receiver also negotiate a specified frequency which is used for transmission of the signal sequence preliminarily. The transmitter transmits the signal sequence using the specified frequency. In the receiver, the matched filter has a tap coefficient corresponding to the signal sequence. When the receiver receives a signal corresponding to the signal sequence, a power level of an output signal from the matched filter indicates high. Therefore, the receiver acquires a start timing of transmitted signal by the matched filter.
In this method, the receiver needs to know the specified frequency to transmit a signal preliminarily. If the receiver knows the specified frequency, this method can establish a timing synchronization between the transmitter and the receiver. The precision of the timing synchronization is equal to a reciprocal of a bandwidth of the spread code. However, in the cognitive radio systems, a transmitter selects a specific frequency which is currently not used to send a signal on every transmission. Therefore, it is difficult for a receiver to predict which frequency is used to send a signal from the transmitter on each transmission.
On the other hand, another conventional timing synchronization method is disclosed in JP-A 2008-177914(KOKAI). This method does not need a negotiation of a specified frequency to transmit a signal between a transmitter and a receiver. According to the method, the transmitter transmits a data signal which is spread by a spread code with using a frequency. The transmitter also transmits the spread code with using another frequency. Both frequencies are Δf apart. The receiver shifts the frequencies of the received signals by the frequency gap of Δf, and multiplies it with the received signal itself. Then, the receiver obtains a de-spread signal at DC. In this method, the receiver can demodulate the received signal without information of the specified frequency, only with the frequency gap of Δf.
However, while the received signal has a wide bandwidth because it is spread by the spread code, the signal at DC has a narrow bandwidth because it is de-spread. If the receiver tries to establish a timing synchronization by using the de-spread signal, the precision of the timing synchronization is low. Therefore, the precision is rougher than the time duration of a symbol in signal of spread code.